Nip outside at sunset tonight, just look up at the sky. The full moon may look a little larger. The preshow buzz calls it the supermoon. NPR's Richard Harris is on the case.

RICHARD HARRIS, BYLINE: All full moons are not created equal. Astronomer Andrew Fraknoi explains that's because the moon's orbit around the earth isn't a perfect circle. It's an ellipse. And tonight, we're in luck.

ANDREW FRAKNOI: We will have moon closest to the Earth at the exact moment or within a minute or two of when it becomes full. And this has no cosmic danger or significance, but it means that the moon will be a little bit brighter and a little bit bigger in our sky.

Fraknoi, who is at Foothill College in the San Francisco Bay Area, says this supermoon is a good excuse to take a romantic stroll. And for the best Hollywood effect, head out around sunset, when the moon is close to the horizon.

When you look at the moon on the horizon, especially when there are like buildings in the distance, it looks huge. And because this supermoon will be a tiny bit bigger, it's going to be an especially interesting moon illusion this Saturday night.

HARRIS: The moon illusion is simply a trick of the eye, but a convincing one. And beach-goers know that full tides are higher around full moons. And the supermoon does add a bit of an extra tug, since it's a little bit closer to earth than usual.

FRAKNOI: But the difference between it being a little bit closer in its orbit or a little bit further is only a question of about an inch in the height of the water.

HARRIS: So if you were expecting a supermoon to rock your world, sorry. You'll have better luck waiting for a giant asteroid to smash into our planet. But there is a darker side to this story. Fraknoi says it used to be that events like supermoons and planetary conjunctions were 100 percent happy news.

FRAKNOI: But now more and more, I think partly because of tabloid television, when something is announced as happening in the sky, it leads people to be afraid. And astronomer David Morrison has coined this new phrase called cosmophobia.

HARRIS: Astronomers will tell you that the moon does not actually have a dark side, despite what we learned from Pink Floyd. But the human mind apparently does.

(SOUNDBITE OF SONG, "YOU HUNG THE MOON")

ELVIS COSTELLO: (Singing) You hung the moon from a gallows in the sky.

HARRIS: Richard Harris, NPR News.

(SOUNDBITE OF SONG, "YOU HUNG THE MOON")

COSTELLO: (Singing) Choked out the light in his blue lunar eye. The shore is a parchment, the sea has no tide since he was taken from my...

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